It is well known to apply disposable lids to drinking cups for carry out sales of beverages, such as coffee. Such lids keep the liquid drink in the cup and prevent heat loss out to the surrounding environment. Many commonly used lids have a small drinking hole formed in the top surface so the user can drink from the cup without removing the lid. For example, U.S. Pat. No. 4,589,569 discloses a lid having a hole formed in its top wall, along with a recess formed in the top wall to accommodate the upper lip of the user. The hole is made relatively small to inhibit accidental splashing or spilling of the liquid through the drinking opening.
Conventional lids with drinking holes have to strike a balance between spillage and drinking flow. As the hole is made smaller, the amount of spillage prevented by the lid increases, but the amount of liquid flow through the hole as the user takes a drink decreases. Thus, by reducing the size of the hole, spillage is reduced at the expense of drinking flow. If the hole is too small, a comfortable amount of drinking flow cannot be achieved. If the hole is made larger to achieve better drinking flow, more liquid can accidentally spill or splash out through the enlarged hole. Prior art lid designs typically include longer side walls so that the upper wall of the lid is well above the top rim of the cup. However, liquid still spills out through the drinking hole.
There are three basic types of liquid spillage from lidded cups: vertical, horizontal and swirling. Vertical spillage can be caused by the cup being vertically slammed too hard on a rigid surface. The liquid in the cup experiences negative gravity acceleration and shoots vertically up to the lid. Any liquid aligned with the drinking hole shoots out of the cup through the hole. Horizontal spillage occurs when the cup is horizontally displaced. The horizontal motion causes a wave of liquid to ride up the side of the cup, along the upper wall of the lid, and out the drinking hole. Swirling spillage occurs when arcing translational movement of the cup causes a wave of fluid to ride up the side of the cup and along the circumference of the cup and lid. When the wave reaches the drinking hole, liquid splashes out from the cup through the hole.
Because of the trade off between spillage prevention and drinking flow, prior art lid designs simply cannot provide adequate spillage prevention, while still providing adequate drinking flow when the user tips the cup for a drink. Raising the top wall of the lid well above the rim of the cup does little to abate the vertical, horizontal and swirling waves of liquid that are responsible for most accidental spillage. There is a need for a lid design that improves spillage protection without comprising the desired drinking flow rate through the lid.